Combined effects of light intensity and NH_4^+-enrichment on growth, pigmentation, and photosynthetic performance of Ulva prolifera(Chlorophyta)

The aim of this study was to investigate the effects of light intensity and enhanced nitrogen supply on the growth and photosynthesis of the green-tide macroalga, Ulvaprolifera. Thalli of U. prolifera were grown in natural or NH^-enriched seawater under two different light intensities for 7 days, and then the growth rate, pigmentation, and photosynthetic performance of the thalli were evaluated. The results show that the relative growth rate （RGR） was markedly higher under the high light level than under the low light level. Enrichment with NH~ enhanced the RGR under high light intensity, but did not affect RGR under low light intensity. In low light conditions, NH;-enrichment resulted in a marked decrease in the maximal photosynthetic rate （Pro） and the maximum carbon fixation rate （Vmax）, but it did not affect the half saturation constant for carbon （K0.5） or the ratio of Vmax to K0.5, which reflects the carbon acquisition efficiency. In high light conditions, Pm, K05, and the dark respiration rate （Rd） increased under NHI enrichment, but Vmax and the Vmax/Ko5 ratio decreased. Regardless of the light intensity, NH^＋4-enrichment did not affect the apparent photosynthetic efficiency （a）, which reflects the ability of the alga to use light energy at low light levels. Under both low and high light intensities, the chlorophyll a （Chl a）, chlorophyll b （Chl b）, and carotenoids （Car） contents in thalli were higher in NH1-enriched than in natural seawater, except that there was a decrease in the Chl b content of thalli in NH1-enriched seawater under low light intensity. Therefore, NH^＋4 enrichment improved the growth and photosynthetic performance of U. prolifera under high light intensity, but not under low light intensity. We discuss the possible mechanisms underlying these physiological responses.